Heretofore, an epoxy resin composition has been used as a material for encapsulating photosemiconductor elements such as a photodetector or a light emittor in view of superiority in transparency, moisture resistance and heat of such a composition.
However, in spite of superiority in transparency, a conventional epoxy resin composition has the defect that mold releasability drastically deteriorates when forming a photosemiconductor device by transfer molding a photosemiconductor element and an epoxy resin composition in a forming die. For this reason, various kinds of problems are caused. For example, a photosemiconductor device deforms, a package cracks, a photosemiconductor element and an encapsulating resin separate, or gold wires peel off from stress in mold releasing. Such problems are distinctly identified by a variety of life tests for photosemiconductor devices, however, generally such photosemiconductor devices are judged as insufficient before they are subjected to such life tests and treated separately as defectives.
For this reason, heretofore, countermeasures have been studied. For example, there is a method that a conventional mold releasing agent is preliminarily mixed in an epoxy resin composition as an encapsulating material. The addition of some kinds of the mold releasing agent to an epoxy resin composition makes it possible to produce a resin composition superior in transparency which is one of the important properties for a resin composition for encapsulating a photosemiconductor element. However, a resin composition superior both in transparency and mold releasability has not been developed. To the contrary, a resin composition for encapsulating which is superior in mold releasability can be produced by the addition of a mold releasing agent, but, however, causes a problem of deterioration in transparency. Thus, it is a current situation that an epoxy resin composition for encapsulating superior both in transparency and mold releasability has not been obtained.